1. Field of the Invention
The present invention relates generally to methods and apparatus for removing cuttings from a deviated wellbore, and more particularly, to methods and apparatus for diverting drilling fluid into a wellbore annulus to remove cuttings from a deviated wellbore as drilling progresses.
2. Description of the Related Art
Historically, oil and gas were produced from hydrocarbon formations by drilling a substantially vertical wellbore from a surface location above the formation to the desired hydrocarbon zone at some depth below the surface. Modern drilling technology and techniques allow for the drilling of wellbores that deviate from vertical. In particular, deviated or horizontal wellbores may be drilled from a convenient surface location to the desired hydrocarbon zone. It is also common to drill “sidetrack” boreholes within existing wellbores to access other hydrocarbon formations.
During such drilling operations, it may be economically infeasible to use jointed drill pipe. Therefore, tools and methods have been developed for drilling wellbores using coiled tubing, which is a single length of continuous, unjointed tubing spooled onto a reel for storage in sufficient quantities to exceed the length of the wellbore. A typical drilling operation is depicted in FIG. 1, which includes a coiled tubing system 100 on the surface 10 and a drilling assembly 200 shown drilling a subsurface deviated wellbore 170. The coiled tubing system 100 includes a power supply 110, a surface processor 120, and a coiled tubing spool 130. An injector head unit 140 feeds and directs the coiled tubing 150 from the spool 130 into the well 160. The drilling assembly 200, which includes a drilling motor 205 and a drill bit 210, connects to the lower end of the coiled tubing 150 and extends into the deviated wellbore 170 being drilled.
The drilling motor 205 operates the drill bit 210, which cuts into the wellbore wall 175, thereby creating cuttings 180 that tend to accumulate in the wellbore annulus 165 formed between the coiled tubing 150 and the wall 175 of the deviated wellbore 170. The drilling motor 205 is powered by drilling fluid pumped from the surface 10 through the coiled tubing 150. The drilling fluid flows through the drilling motor 205, out through the drill bit 210, and into the wellbore annulus 165 back up to the surface 10.
When using drill pipe that rotates during the drilling process, cuttings 180 do not tend to accumulate in the annular area 165 of the wellbore 170. The rotation of the pipe working against the cuttings 180 tends to stir up the cuttings 180 so that they are more easily carried away by the drilling fluid as it flows through the wellbore annulus 165 to the surface 10. However, when drilling using coiled tubing 150, which does not rotate, the cuttings 180 tend to accumulate in the wellbore annulus 165 and may even bury the coiled tubing 150. Therefore, when using coiled tubing 150 to drill a deviated wellbore 170, it is particularly important for the drilling fluid to flow through the wellbore annulus 165 at a velocity sufficient to lift the cuttings 180 and carry them back to the surface 10. However, the components of the drilling assembly 200 have smaller internal diameters than the coiled tubing 150, so excessive drilling fluid velocities must be avoided to prevent erosion or abrasion of the internal components of the drilling assembly 200.
Thus, one method for removing cuttings 180 from a deviated wellbore 170 is to periodically perform wiper trips. To conduct a wiper trip, drilling is halted, and the coiled tubing 150 is pulled to drag the drilling assembly 200 through the previously drilled wellbore 170 to stir up the cuttings 180 so that the drilling fluid can carry those cuttings 180 back to the surface 10. Wiper trips are undesirable because they consume valuable drilling time and can cause damage to the components of the drilling assembly 200, such as the drill bit 210.
U.S. Pat. No. 5,984,011 to Misselbrook et al., hereby incorporated herein by reference for all purposes, discloses another method for removing cuttings from a deviated wellbore without using wiper trips. The method includes ceasing drilling, pumping fluid into the wellbore at a critical level of flow that exceeds the drilling flow rate, and valving at least a portion of the fluid to bypass the drilling motor, preferably in the vicinity of the drilling motor.
Misselbrook teaches that drilling is ceased so that additional cuttings are not generated while removing the existing cuttings from the wellbore. The critical level of flow is typically 3–5 feet/second, or at least 120% of the drilling flow rate, and possibly up to 150% of the drilling flow rate. At the critical level of flow, approximately 60 linear feet/minute can be cleared without drilling as compared to a wiper trip, which typically does not proceed at a rate greater than 50 feet/minute, and usually proceeds slower. Further, with drilling ceased, the weight-on-bit can be managed to cause the coiled tubing to helix or cork screw within the wellbore, thereby lifting substantial portions of the coiled tubing off the wellbore wall to enhance cutting removal. In summary, the Misselbrook method includes ceasing drilling, opening a valve, and increasing the flow rate to a critical level to bypass the drilling motor and sweep out any cuttings that have accumulated in the wellbore. The cutting removal phase may be enhanced by helixing the coiled tubing within the wellbore.
U.S. Pat. No. 5,979,572 to Boyd et al., hereby incorporated herein by reference for all purposes, discloses a by-pass valving apparatus that enables removal of cuttings from a wellbore drilled using either conventional drill pipe or coiled tubing. The valving arrangement comprises an outer body with an inner spool mounted therein, motion control means to effect uni-directional rotation of the spool through pre-set positions, and a spring that biases the spool to a closed-off position. Fluid pumped through the drill string from the surface moves the spool against the spring, while simultaneously; the motion control means causes the spool to rotate to a pre-set position. Relieving the fluid pressure causes the spool to move axially with the spring and to rotate via the motion control means to the closed-off position. Subsequent pumping of fluid through the drill string causes the spool to move axially and to rotate to yet another pre-set position. In this way, the spool is selectively moved through a number of pre-set positions that close off flow, or direct fluid entirely or partially into the wellbore.
Boyd teaches that except during drilling, it is desirable to suspend operation of the drill motor and telemetry equipment to prolong its useful operating life. Therefore, the by-pass valving arrangement is positioned upstream of the motor and telemetry equipment so that fluid may be circulated into the wellbore while bypassing the drilling equipment. In circumstances where the bit might become stuck in the hole, the flow may be partially by-passed through the valving arrangement so that a reduced flow rotates the drill motor at a slower rate. Boyd states that use of the flow control tool allows for increased mud flow rates during circulating operations, thereby reducing the mud circulating time and increasing the removal efficiency of the cuttings. Further, use of the tool provides an increased motor life since not all of the mud flowing at the higher circulating rates must pass through the motor.
The apparatus and methods disclosed by Misselbrook and Boyd each eliminate the need for wiper trips, but each recommends disrupting drilling to sweep the wellbore clean of cuttings. Thus, it would be desirable to provide a cutting removal apparatus and method that does not disrupt drilling. Accordingly, it would be desirable to provide a continuous cutting removal apparatus and method that operates while drilling proceeds.
The present invention overcomes the deficiencies of the prior art.